Tank explosions in bulk plant installations handling volatile materials such as liquified petroleum gas and liquid ammonia may be caused by the inability of a tank's safety relief valve system to relieve enough pressure at an adequate rate. Tank failure may be caused by an intense fuel-fed fire surrounding the tank for an extended period. As the tank continues to be heated by the fire, the steel in the unwetted portion of the tank may be weakened by overheating, and thereby, cause tank failure.
These fires may be caused by excessive spills of materials such as liquified petroleum (LP) gas, which is easily ignited. Once ignited, the fire can eventually heat the tank enough to cause tank failure if the spilling of the material is left unchecked. These spills are generally caused by breaks or cracks in the transfer line or hose, as when a driver moves a truck away from the point of filling without disconnecting the hose. An excess flow valve in an internal flow control valve can shut off the flow of the escaping material if the break is large enough to actuate the excess flow shut-off valve. But should there be only a small crack or break in the material transfer line, large enough to create a serious hazard, but not large enough to shut the excess flow valve, the material will continue to leak and may cause a fire and possibly an explosion.
Several methods have been proposed for insuring safety in emergency situations such as those mentioned above in bulk plants handling LP gas or liquid ammonia (NH.sub.3). One proposed method utilized an elaborate sprinkler system to keep tanks containing volatile fuels cool in the event of fire in an attempt to prevent tank failure. Should a fire occur near the tanks, water would be sprayed on the tanks until the fire could be extinguished. This proposed system lacks any provision to terminate the flow of material fueling the fire.
Another method was to bury the storage tanks to prevent fire from reaching the external portions of the tank. This method is costly and only treats the effect of the emergency situation and not the cause.
A third proposed method was to cover above ground storage tanks with a heat resistant material. Since the tanks are relatively large, this proposal was relatively costly, and, in addition, did not obviate the cause of the fire or stop the flow of fuel feeding such fire.
The foregoing proposals did not correct the emergency situation. The use of a quick-closing valve, as shown in St. Clair U.S. Pat. No. 2,538,806 or a safety flow valve as shown in Dyer et al. U.S. Pat. No. 3,164,359 in association with a storage tank would not solve the problem resulting from an emergency situation in the transfer line or hose external of the storage tank.